Abacus: online shop orders, Stripe.com payments

[mrtnzlml]

The goal of this task is to be able to accept orders and online payments in our shop. Everything except the actual payments processing is done by us. Payments are done via https://stripe.com/en-mx/payments

There are multiple parts that must be done:

Abacus KO

• be able to add products to the cart (take into account availability/stock)
• send the selected products via "create checkout session" mutation to BE and redirect to Stripe.com
• create "success" and "cancel" pages to be used by Stripe.com for final redirecting

Abacus

• prepare "create checkout session" mutation (validate prices and availability), return Stripe.com pay URL
• save order details and provide a query to be able to fetch the orders for processing
• prepare mutations to be able to update the order details (order fulfillment)
• prepare webhooks for Stripe.com to be able to receive payment updates
• prepare a way how to change the stocks (add new items, remove items from the stock)
• send relevant emails when appropriate

Abacus BO

• fetch all orders and display the ones that need to be fulfilled
• add ability to mark the orders as fulfilled

Legals

• refund and return policy
• terms of service
• privacy policy

After checks

• fix all clippy errors
• make sure it works
• update abacus readmes